The invention relates to the field of intravascular balloon catheters, and more particularly to a catheter balloon having a stepped outer diameter that provides for improved dilatation and stenting.
In percutaneous transluminal coronary angioplasty (PTCA) procedures a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient""s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter, having an inflatable balloon on the distal portion thereof, is advanced into the patient""s coronary anatomy over the previously introduced guidewire until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. at least about 8 atmospheres) so that the stenosis is compressed against the arterial wall to open up the passageway. Preferably, the inflated diameter of the working length of the balloon. is approximately the same as the native diameter of the body lumen being dilated, so as to complete the dilatation but not overexpand the artery wall. However, damage to the vessel wall at and around the stenosis can result from the expansion of the balloon against the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated vessel and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. Further details of stents and stent delivery systems can be found in U.S. Pat. No. 5,507,768 (Lau et al.), U.S. Pat. No. 5,458,615 (Klemm et al.), and U.S. Pat. No. 5,514,154 (Lau et al.), which are incorporated herein by reference in their entireties. Thus, stents are used to open a stenosed vessel, and strengthen the dilated area by remaining inside the vessel. Although stents have been used for some time, the effectiveness of a stent can be diminished if it is not properly implanted within the vessel. One difficulty has been deploying the stent at the desired location in the vessel and completely expanding the stent during the deployment.
Therefore, what has been needed is an improved balloon catheter with a balloon which expands to dilatate a stenosis or to deploy a stent within the patient. The present invention satisfies these and other needs.
The invention is directed to a balloon catheter with a balloon having a stepped outer diameter formed by a plurality of sections having different outer diameters, and to a stent delivery system with a stent disposed about and mounted on the balloon.
The balloon catheter of the invention generally comprises a catheter having an elongated shaft with an inflatable balloon on a distal section of the catheter shaft. The balloon may be configured for dilatation, or for stent delivery with a stent disposed about and mounted on a working length of the balloon. In one embodiment, the balloon has first and second tapered sections adjacent the distal and proximal ends of the working length of the balloon, respectively, and third and fourth tapered sections adjacent the first and second tapered section, respectively. The first and second tapered sections taper at a first angle and a second angle, respectively, to a smaller outer diameter than the inflated outer diameter of the working length inflated within a deployment range of the balloon. The third tapered section tapers at a third angle, larger than the first angle, to a smaller inflated outer diameter than the inflated outer diameter of the first tapered section inflated within the deployment pressure range of the balloon, and the fourth tapered section tapers at a fourth angle, larger than the second angle, to a smaller inflated outer diameter than the inflated outer diameter of the second tapered section inflated within the deployment pressure range of the balloon. The deployment range is the inflation pressure at which the balloon working section is intended to be expanded within the patient to expand and deploy the stent. Below the deployment range, the inflation pressure is insufficient to expand the working length of the balloon and stent thereon. Above the deployment range, the balloon may rupture as the pressure approaches the burst pressure of the balloon. The first angle and the second angle are relatively small so that the inflated outer diameter of the first and second tapered sections is not significantly less than the working length inflated outer diameter. Consequently, the first and second tapered sections can be inflated to dilatate a stenosis or expand an end of a stent which extends somewhat beyond the end of the working length of the balloon. However, the small angle of the first and second tapered sections is such that a sharp transition section on the balloon which could produce sheer forces against the vessel wall at the junction between the working length and the first tapered sections is avoided. Additionally, the length of the first and second tapered sections is relatively small, and the third and fourth tapered sections taper at a relatively large angle to a smaller outer diameter, to thereby avoid the potential damage to the vessel wall caused by the proximal and distal ends of the balloon beyond the ends of the working length expanding against the vessel wall.
In another embodiment, the balloon has expandable retention sections proximal and distal to the working length of the balloon. With a stent in place on the working length of the balloon, the retention sections inflate together at low pressure before the working section significantly expands, so that the expanded retention sections form a barrier at either end of the stent to inhibit the longitudinal displacement of the stent on the balloon. The expanded outer diameter of the retention sections, at a low pressure less than the deployment range of the balloon, is at least about 200% greater than the unexpanded outer diameter of the stent on the balloon prior to expansion of the working length of the balloon. As the inflation pressure is increased within the deployment range of the balloon, the working length of the balloon with the stent thereon will expand. The outer diameter of the retention sections is at least 30% less than the outer diameter of the working length expanded within the deployment range of the balloon, so that they form a relatively small diameter portion, which thereby minimizes potential damage to the vessel wall caused by the expansion of the proximal and distal ends of the balloon against the vessel wall.
The balloon is preferably formed of a semi or low compliant material, such as polyamides including nylon and PEBAX, and polyurethanes. The term xe2x80x9ccompliantxe2x80x9d as used herein refers to thermosetting and thermoplastic polymers which exhibit substantial radial growth upon the application of radially expansive force. The radial growth of a balloon formed of a noncompliant material such as PET is typically less than about 0.02 mm/ATM, compared to about 0.025 to about 0.045 mm/ATM for a balloon formed of low compliant material such as nylon 12. In a presently preferred embodiment, the balloon is preformed in a mold, so that the working length, tapered sections, and retention sections of the balloon have predictable inflated outer diameters which form when the balloon is inflated within a deployment range of the balloon.
The balloon catheter of the invention provides for improved focal expansion and stenting due to the tapered sections proximal and distal to the working length of the balloon. Moreover, in the embodiment having the retention sections, stent migration on the balloon and the potential for damage to the vessel wall is minimized. These and other advantages of the invention will become more apparent from the following detailed description of the invention and the accompanying exemplary drawings.